Conventionally, an ad-hoc network has been known which is a radio network autonomously constituted by multiple radio terminals having a relay function. Additionally, a method by which a radio terminal constituting the ad-hoc network communicates with a radio base station has been proposed (See Patent Document 1, for example). To be specific, the radio terminal communicates with the radio base station by way of another radio terminal.
More specifically, multiple radio terminals constitute a tree-structured network in which a radio terminal directly communicating with a radio base station is set to be a root radio terminal. Since a lower-level radio terminal communicates with the radio base station by way of an upper-level radio terminal, the lower-level radio terminal can communicate with the radio base station even when the lower-level radio terminal is outside of the radio wave reachable range of the radio base station.    Patent Document 1: Japanese Patent Application No. 2003-124876 (Pages 6-7, FIG. 7)